The proposed research is a continuation of work already underway and is a logical extension of my prior investigations concerning the origin and nature of the centrifugal fibers of the eighth nerve. The primary objective is to gather conclusive evidence concerning the cells of origin of the efferent terminal in the organ of Corti by use of 1) a histochemical neuron labeling method involving retrograde axonal transport of horseradish peroxidase (HRP) and 2) an autoradiographic technique for labeling terminals or entire neurons of small neuronal pools through somal uptake of tritiated amino acids and transport of synthesized proteins into all neuronal parts. The two methods are complementary and must yield results which can be interpreted similarly if findings are correct. Cochleas of weanling guinea pigs will be perfused through a closed system with HRP. After varying survival times, cochleas will be prepared for ultrastructural, histochemical demonstration of HRP; brain stems will be sectioned and prepared histochemically for light microscopical study. As our pilot work shows that retrograde transport does not occur under ordinary circumstances in eighth nerve fibers, variations in the method will be attempted (i.e., axonal injury). For autoradiographic research, microliter quantities of 3H-amino acids will be injected into the spiral ganglion for somal uptake by small neuronal pools. (Spiral ganglion cells are a possible source of the efferent terminals in the organ of Corti.) Animals will be sacrificed at intervals from several minutes to many hours. Cochleas will be prepared for autoradiographic electron microscopy to determine 1) the specific type and exact location of terminals of labeled neurons and 2) whether or not entire neurons wil be labeled. Findings should help to resolve the question of the source of the efferent terminals in the organ of Corti and should provide an in-depth appraisal of the HRP techniques as applied to the inner ear. Data concerning barriers to HRP movement and transport in the cochlea may be clinically relevant with respect to the movement of noxious agent.